3 * This program is free software; you can redistribute it and/or modify
4 * it under the terms of the GNU General Public License as published by
5 * the Free Software Foundation; either version 2 of the License, or
6 * (at your option) any later version.
8 * This program is distributed in the hope that it will be useful,
9 * but WITHOUT ANY WARRANTY; without even the implied warranty of
10 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
11 * GNU General Public License for more details.
13 * You should have received a copy of the GNU General Public License along
14 * with this program; if not, write to the Free Software Foundation, Inc.,
15 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
16 * http://www.gnu.org/copyleft/gpl.html
20 * @author Aaron Schulz
24 * Multi-datacenter aware caching interface
26 * All operations go to the local cache, except the delete()
27 * and touchCheckKey(), which broadcast to all clusters.
28 * This class is intended for caching data from primary stores.
29 * If the get() method does not return a value, then the caller
30 * should query the new value and backfill the cache using set().
31 * When the source data changes, the delete() method should be called.
32 * Since delete() is expensive, it should be avoided. One can do so if:
33 * - a) The object cached is immutable; or
34 * - b) Validity is checked against the source after get(); or
35 * - c) Using a modest TTL is reasonably correct and performant
36 * Consider using getWithSetCallback() instead of the get()/set() cycle.
38 * Instances of this class must be configured to point to a valid
39 * PubSub endpoint, and there must be listeners on the cache servers
40 * that subscribe to the endpoint and update the caches.
42 * Broadcasted operations like delete() and touchCheckKey() are done
43 * synchronously in the local cluster, but are relayed asynchronously.
44 * This means that callers in other datacenters will see older values
45 * for a however many milliseconds the datacenters are apart. As with
46 * any cache, this should not be relied on for cases where reads are
47 * used to determine writes to source (e.g. non-cache) data stores.
49 * All values are wrapped in metadata arrays. Keys use a "WANCache:" prefix
50 * to avoid collisions with keys that are not wrapped as metadata arrays. The
51 * prefixes are as follows:
52 * - a) "WANCache:v" : used for regular value keys
53 * - b) "WANCache:s" : used for temporarily storing values of tombstoned keys
54 * - c) "WANCache:t" : used for storing timestamp "check" keys
59 class WANObjectCache
{
60 /** @var BagOStuff The local cluster cache */
62 /** @var string Cache pool name */
64 /** @var EventRelayer */
68 protected $lastRelayError = self
::ERR_NONE
;
70 /** Seconds to tombstone keys on delete() */
71 const HOLDOFF_TTL
= 10;
72 /** Seconds to keep dependency purge keys around */
73 const CHECK_KEY_TTL
= 31536000; // 1 year
74 /** Seconds to keep lock keys around */
76 /** Default remaining TTL at which to consider pre-emptive regeneration */
78 /** Default TTL for temporarily caching tombstoned keys */
81 /** Idiom for set()/getWithSetCallback() TTL */
83 /** Idiom for getWithSetCallback() callbacks to avoid calling set() */
84 const TTL_UNCACHEABLE
= -1;
86 /** Cache format version number */
89 /** Fields of value holder arrays */
90 const FLD_VERSION
= 0;
95 /** Possible values for getLastError() */
96 const ERR_NONE
= 0; // no error
97 const ERR_NO_RESPONSE
= 1; // no response
98 const ERR_UNREACHABLE
= 2; // can't connect
99 const ERR_UNEXPECTED
= 3; // response gave some error
100 const ERR_RELAY
= 4; // relay broadcast failed
102 const VALUE_KEY_PREFIX
= 'WANCache:v:';
103 const STASH_KEY_PREFIX
= 'WANCache:s:';
104 const TIME_KEY_PREFIX
= 'WANCache:t:';
106 const PURGE_VAL_PREFIX
= 'PURGED:';
109 * @param array $params
110 * - cache : BagOStuff object
112 * - relayer : EventRelayer object
114 public function __construct( array $params ) {
115 $this->cache
= $params['cache'];
116 $this->pool
= $params['pool'];
117 $this->relayer
= $params['relayer'];
121 * @return WANObjectCache Cache that wraps EmptyBagOStuff
123 public static function newEmpty() {
124 return new self( array(
125 'cache' => new EmptyBagOStuff(),
127 'relayer' => new EventRelayerNull( array() )
132 * Fetch the value of a key from cache
134 * If passed in, $curTTL is set to the remaining TTL (current time left):
135 * - a) INF; if the key exists, has no TTL, and is not expired by $checkKeys
136 * - b) float (>=0); if the key exists, has a TTL, and is not expired by $checkKeys
137 * - c) float (<0); if the key is tombstoned or existing but expired by $checkKeys
138 * - d) null; if the key does not exist and is not tombstoned
140 * If a key is tombstoned, $curTTL will reflect the time since delete().
142 * The timestamp of $key will be checked against the last-purge timestamp
143 * of each of $checkKeys. Those $checkKeys not in cache will have the last-purge
144 * initialized to the current timestamp. If any of $checkKeys have a timestamp
145 * greater than that of $key, then $curTTL will reflect how long ago $key
146 * became invalid. Callers can use $curTTL to know when the value is stale.
147 * The $checkKeys parameter allow mass invalidations by updating a single key:
148 * - a) Each "check" key represents "last purged" of some source data
149 * - b) Callers pass in relevant "check" keys as $checkKeys in get()
150 * - c) When the source data that "check" keys represent changes,
151 * the touchCheckKey() method is called on them
153 * For keys that are hot/expensive, consider using getWithSetCallback() instead.
155 * @param string $key Cache key
156 * @param mixed $curTTL Approximate TTL left on the key if present [returned]
157 * @param array $checkKeys List of "check" keys
158 * @return mixed Value of cache key or false on failure
160 final public function get( $key, &$curTTL = null, array $checkKeys = array() ) {
162 $values = $this->getMulti( array( $key ), $curTTLs, $checkKeys );
163 $curTTL = isset( $curTTLs[$key] ) ?
$curTTLs[$key] : null;
165 return isset( $values[$key] ) ?
$values[$key] : false;
169 * Fetch the value of several keys from cache
171 * @see WANObjectCache::get()
173 * @param array $keys List of cache keys
174 * @param array $curTTLs Map of (key => approximate TTL left) for existing keys [returned]
175 * @param array $checkKeys List of "check" keys
176 * @return array Map of (key => value) for keys that exist
178 final public function getMulti(
179 array $keys, &$curTTLs = array(), array $checkKeys = array()
184 $vPrefixLen = strlen( self
::VALUE_KEY_PREFIX
);
185 $valueKeys = self
::prefixCacheKeys( $keys, self
::VALUE_KEY_PREFIX
);
186 $checkKeys = self
::prefixCacheKeys( $checkKeys, self
::TIME_KEY_PREFIX
);
188 // Fetch all of the raw values
189 $wrappedValues = $this->cache
->getMulti( array_merge( $valueKeys, $checkKeys ) );
190 $now = microtime( true );
192 // Get/initialize the timestamp of all the "check" keys
193 $checkKeyTimes = array();
194 foreach ( $checkKeys as $checkKey ) {
195 $timestamp = isset( $wrappedValues[$checkKey] )
196 ? self
::parsePurgeValue( $wrappedValues[$checkKey] )
198 if ( !is_float( $timestamp ) ) {
199 // Key is not set or invalid; regenerate
200 $this->cache
->add( $checkKey,
201 self
::PURGE_VAL_PREFIX
. $now, self
::CHECK_KEY_TTL
);
205 $checkKeyTimes[] = $timestamp;
208 // Get the main cache value for each key and validate them
209 foreach ( $valueKeys as $vKey ) {
210 if ( !isset( $wrappedValues[$vKey] ) ) {
211 continue; // not found
214 $key = substr( $vKey, $vPrefixLen ); // unprefix
216 list( $value, $curTTL ) = $this->unwrap( $wrappedValues[$vKey], $now );
217 if ( $value !== false ) {
218 $result[$key] = $value;
219 foreach ( $checkKeyTimes as $checkKeyTime ) {
220 // Force dependant keys to be invalid for a while after purging
221 // to reduce race conditions involving stale data getting cached
222 $safeTimestamp = $checkKeyTime + self
::HOLDOFF_TTL
;
223 if ( $safeTimestamp >= $wrappedValues[$vKey][self
::FLD_TIME
] ) {
224 $curTTL = min( $curTTL, $checkKeyTime - $now );
229 $curTTLs[$key] = $curTTL;
236 * Set the value of a key from cache
238 * Simply calling this method when source data changes is not valid because
239 * the changes do not replicate to the other WAN sites. In that case, delete()
240 * should be used instead. This method is intended for use on cache misses.
242 * @param string $key Cache key
243 * @param mixed $value
244 * @param integer $ttl Seconds to live [0=forever]
245 * @return bool Success
247 final public function set( $key, $value, $ttl = 0 ) {
248 $key = self
::VALUE_KEY_PREFIX
. $key;
249 $wrapped = $this->wrap( $value, $ttl );
251 $func = function ( $cache, $key, $cWrapped ) use ( $wrapped ) {
252 return ( is_string( $cWrapped ) )
253 ?
false // key is tombstoned; do nothing
257 return $this->cache
->merge( $key, $func, $ttl, 1 );
261 * Purge a key from all clusters
263 * This should only be called when the underlying data (being cached)
264 * changes in a significant way. This deletes the key and starts a hold-off
265 * period where the key cannot be written to for a few seconds (HOLDOFF_TTL).
266 * This is done to avoid the following race condition:
267 * a) Some DB data changes and delete() is called on a corresponding key
268 * b) A request refills the key with a stale value from a lagged DB
269 * c) The stale value is stuck there until the key is expired/evicted
271 * This is implemented by storing a special "tombstone" value at the cache
272 * key that this class recognizes; get() calls will return false for the key
273 * and any set() calls will refuse to replace tombstone values at the key.
274 * For this to always avoid writing stale values, the following must hold:
275 * a) Replication lag is bounded to being less than HOLDOFF_TTL; or
276 * b) If lag is higher, the DB will have gone into read-only mode already
278 * If called twice on the same key, then the last hold-off TTL takes
279 * precedence. For idempotence, the $ttl should not vary for different
280 * delete() calls on the same key. Also note that lowering $ttl reduces
281 * the effective range of the 'lockTSE' parameter to getWithSetCallback().
283 * @param string $key Cache key
284 * @param integer $ttl How long to block writes to the key [seconds]
285 * @return bool True if the item was purged or not found, false on failure
287 final public function delete( $key, $ttl = self
::HOLDOFF_TTL
) {
288 $key = self
::VALUE_KEY_PREFIX
. $key;
289 // Avoid indefinite key salting for sanity
290 $ttl = max( $ttl, 1 );
291 // Update the local cluster immediately
292 $ok = $this->cache
->set( $key, self
::PURGE_VAL_PREFIX
. microtime( true ), $ttl );
293 // Publish the purge to all clusters
294 return $this->relayPurge( $key, $ttl ) && $ok;
298 * Fetch the value of a timestamp "check" key
300 * The key will be *initialized* to the current time if not set,
301 * so only call this method if this behavior is actually desired
303 * The timestamp can be used to check whether a cached value is valid.
304 * Callers should not assume that this returns the same timestamp in
305 * all datacenters due to relay delays.
307 * The level of staleness can roughly be estimated from this key, but
308 * if the key was evicted from cache, such calculations may show the
309 * time since expiry as ~0 seconds.
311 * Note that "check" keys won't collide with other regular keys
314 * @return float UNIX timestamp of the key
316 final public function getCheckKeyTime( $key ) {
317 $key = self
::TIME_KEY_PREFIX
. $key;
319 $time = self
::parsePurgeValue( $this->cache
->get( $key ) );
320 if ( $time === false ) {
321 // Casting assures identical floats for the next getCheckKeyTime() calls
322 $time = (string)microtime( true );
323 $this->cache
->add( $key, self
::PURGE_VAL_PREFIX
. $time, self
::CHECK_KEY_TTL
);
324 $time = (float)$time;
331 * Purge a "check" key from all clusters, invalidating keys that use it
333 * This should only be called when the underlying data (being cached)
334 * changes in a significant way, and it is impractical to call delete()
335 * on all keys that should be changed. When get() is called on those
336 * keys, the relevant "check" keys must be supplied for this to work.
338 * The "check" key essentially represents a last-modified field.
339 * It is set in the future a few seconds when this is called, to
340 * avoid race conditions where dependent keys get updated with a
341 * stale value (e.g. from a DB slave).
343 * This is typically useful for keys with static names or some cases
344 * dynamically generated names where a low number of combinations exist.
345 * When a few important keys get a large number of hits, a high cache
346 * time is usually desired as well as lockTSE logic. The resetCheckKey()
347 * method is less appropriate in such cases since the "time since expiry"
348 * cannot be inferred.
350 * Note that "check" keys won't collide with other regular keys
352 * @see WANObjectCache::get()
354 * @param string $key Cache key
355 * @return bool True if the item was purged or not found, false on failure
357 final public function touchCheckKey( $key ) {
358 $key = self
::TIME_KEY_PREFIX
. $key;
359 // Update the local cluster immediately
360 $ok = $this->cache
->set( $key,
361 self
::PURGE_VAL_PREFIX
. microtime( true ), self
::CHECK_KEY_TTL
);
362 // Publish the purge to all clusters
363 return $this->relayPurge( $key, self
::CHECK_KEY_TTL
) && $ok;
367 * Delete a "check" key from all clusters, invalidating keys that use it
369 * This is similar to touchCheckKey() in that keys using it via
370 * getWithSetCallback() will be invalidated. The differences are:
371 * a) The timestamp will be deleted from all caches and lazily
372 * re-initialized when accessed (rather than set everywhere)
373 * b) Thus, dependent keys will be known to be invalid, but not
374 * for how long (they are treated as "just" purged), which
375 * effects any lockTSE logic in getWithSetCallback()
376 * The advantage is that this does not place high TTL keys on every cache
377 * server, making it better for code that will cache many different keys
378 * and either does not use lockTSE or uses a low enough TTL anyway.
380 * This is typically useful for keys with dynamically generated names
381 * where a high number of combinations exist.
383 * Note that "check" keys won't collide with other regular keys
385 * @see WANObjectCache::touchCheckKey()
386 * @see WANObjectCache::get()
388 * @param string $key Cache key
389 * @return bool True if the item was purged or not found, false on failure
391 final public function resetCheckKey( $key ) {
392 $key = self
::TIME_KEY_PREFIX
. $key;
393 // Update the local cluster immediately
394 $ok = $this->cache
->delete( $key );
395 // Publish the purge to all clusters
396 return $this->relayDelete( $key ) && $ok;
400 * Method to fetch/regenerate cache keys
402 * On cache miss, the key will be set to the callback result,
403 * unless the callback returns false. The arguments supplied are:
404 * (current value or false, &$ttl)
405 * The callback function returns the new value given the current
406 * value (false if not present). Preemptive re-caching and $checkKeys
407 * can result in a non-false current value. The TTL of the new value
408 * can be set dynamically by altering $ttl in the callback (by reference).
410 * Usually, callbacks ignore the current value, but it can be used
411 * to maintain "most recent X" values that come from time or sequence
412 * based source data, provided that the "as of" id/time is tracked.
414 * Usage of $checkKeys is similar to get()/getMulti(). However,
415 * rather than the caller having to inspect a "current time left"
416 * variable (e.g. $curTTL, $curTTLs), a cache regeneration will be
417 * triggered using the callback.
419 * The simplest way to avoid stampedes for hot keys is to use
420 * the 'lockTSE' option in $opts. If cache purges are needed, also:
421 * a) Pass $key into $checkKeys
422 * b) Use touchCheckKey( $key ) instead of delete( $key )
423 * Following this pattern lets the old cache be used until a
424 * single thread updates it as needed. Also consider tweaking
425 * the 'lowTTL' parameter.
429 * $key = wfMemcKey( 'cat-recent-actions', $catId );
430 * // Function that derives the new key value given the old value
431 * $callback = function( $cValue, &$ttl ) { ... };
432 * // Get the key value from cache or from source on cache miss;
433 * // try to only let one cluster thread manage doing cache updates
434 * $opts = array( 'lockTSE' => 5, 'lowTTL' => 10 );
435 * $value = $cache->getWithSetCallback( $key, $callback, 60, array(), $opts );
440 * $key = wfMemcKey( 'cat-state', $catId );
441 * // The "check" keys that represent things the value depends on;
442 * // Calling touchCheckKey() on them invalidates "cat-state"
443 * $checkKeys = array(
444 * wfMemcKey( 'water-bowls', $houseId ),
445 * wfMemcKey( 'food-bowls', $houseId ),
446 * wfMemcKey( 'people-present', $houseId )
448 * // Function that derives the new key value
449 * $callback = function() { ... };
450 * // Get the key value from cache or from source on cache miss;
451 * // try to only let one cluster thread manage doing cache updates
452 * $opts = array( 'lockTSE' => 5, 'lowTTL' => 10 );
453 * $value = $cache->getWithSetCallback( $key, $callback, 60, $checkKeys, $opts );
456 * @see WANObjectCache::get()
458 * @param string $key Cache key
459 * @param callable $callback Value generation function
460 * @param integer $ttl Seconds to live for key updates. Special values are:
461 * - WANObjectCache::TTL_NONE : cache forever
462 * - WANObjectCache::TTL_UNCACHEABLE : do not cache at all
463 * @param array $checkKeys List of "check" keys
464 * @param array $opts Options map:
465 * - lowTTL : consider pre-emptive updates when the current TTL (sec)
466 * of the key is less than this. It becomes more likely
467 * over time, becoming a certainty once the key is expired.
468 * - lockTSE : if the key is tombstoned or expired (by $checkKeys) less
469 * than this many seconds ago, then try to have a single
470 * thread handle cache regeneration at any given time.
471 * Other threads will try to use stale values if possible.
472 * If, on miss, the time since expiration is low, the assumption
473 * is that the key is hot and that a stampede is worth avoiding.
474 * Setting this above WANObjectCache::HOLDOFF_TTL makes no difference.
475 * - tempTTL : TTL of the temp key used to cache values while a key is tombstoned.
476 * This avoids excessive regeneration of hot keys on delete() but may
477 * result in stale values.
478 * @return mixed Value to use for the key
480 final public function getWithSetCallback(
481 $key, $callback, $ttl, array $checkKeys = array(), array $opts = array()
483 $lowTTL = isset( $opts['lowTTL'] ) ?
$opts['lowTTL'] : min( self
::LOW_TTL
, $ttl );
484 $lockTSE = isset( $opts['lockTSE'] ) ?
$opts['lockTSE'] : -1;
485 $tempTTL = isset( $opts['tempTTL'] ) ?
$opts['tempTTL'] : self
::TEMP_TTL
;
487 // Get the current key value
489 $cValue = $this->get( $key, $curTTL, $checkKeys ); // current value
490 $value = $cValue; // return value
492 // Determine if a regeneration is desired
493 if ( $value !== false && $curTTL > 0 && !$this->worthRefresh( $curTTL, $lowTTL ) ) {
497 // A deleted key with a negative TTL left must be tombstoned
498 $isTombstone = ( $curTTL !== null && $value === false );
499 // Assume a key is hot if requested soon after invalidation
500 $isHot = ( $curTTL !== null && $curTTL <= 0 && abs( $curTTL ) <= $lockTSE );
502 $lockAcquired = false;
504 // Acquire a cluster-local non-blocking lock
505 if ( $this->cache
->lock( $key, 0, self
::LOCK_TTL
) ) {
506 // Lock acquired; this thread should update the key
507 $lockAcquired = true;
508 } elseif ( $value !== false ) {
509 // If it cannot be acquired; then the stale value can be used
514 if ( !$lockAcquired && ( $isTombstone ||
$isHot ) ) {
515 // Use the stash value for tombstoned keys to reduce regeneration load.
516 // For hot keys, either another thread has the lock or the lock failed;
517 // use the stash value from the last thread that regenerated it.
518 $value = $this->cache
->get( self
::STASH_KEY_PREFIX
. $key );
519 if ( $value !== false ) {
524 if ( !is_callable( $callback ) ) {
525 throw new InvalidArgumentException( "Invalid cache miss callback provided." );
528 // Generate the new value from the callback...
529 $value = call_user_func_array( $callback, array( $cValue, &$ttl ) );
530 // When delete() is called, writes are write-holed by the tombstone,
531 // so use a special stash key to pass the new value around threads.
532 if ( $value !== false && ( $isHot ||
$isTombstone ) && $ttl >= 0 ) {
533 $this->cache
->set( self
::STASH_KEY_PREFIX
. $key, $value, $tempTTL );
536 if ( $lockAcquired ) {
537 $this->cache
->unlock( $key );
540 if ( $value !== false && $ttl >= 0 ) {
541 // Update the cache; this will fail if the key is tombstoned
542 $this->set( $key, $value, $ttl );
549 * Get the "last error" registered; clearLastError() should be called manually
550 * @return int ERR_* constant for the "last error" registry
552 final public function getLastError() {
553 if ( $this->lastRelayError
) {
554 // If the cache and the relayer failed, focus on the later.
555 // An update not making it to the relayer means it won't show up
556 // in other DCs (nor will consistent re-hashing see up-to-date values).
557 // On the other hand, if just the cache update failed, then it should
558 // eventually be applied by the relayer.
559 return $this->lastRelayError
;
562 $code = $this->cache
->getLastError();
564 case BagOStuff
::ERR_NONE
:
565 return self
::ERR_NONE
;
566 case BagOStuff
::ERR_NO_RESPONSE
:
567 return self
::ERR_NO_RESPONSE
;
568 case BagOStuff
::ERR_UNREACHABLE
:
569 return self
::ERR_UNREACHABLE
;
571 return self
::ERR_UNEXPECTED
;
576 * Clear the "last error" registry
578 final public function clearLastError() {
579 $this->cache
->clearLastError();
580 $this->lastRelayError
= self
::ERR_NONE
;
584 * Do the actual async bus purge of a key
586 * This must set the key to "PURGED:<UNIX timestamp>"
588 * @param string $key Cache key
589 * @param integer $ttl How long to keep the tombstone [seconds]
590 * @return bool Success
592 protected function relayPurge( $key, $ttl ) {
593 $event = $this->cache
->modifySimpleRelayEvent( array(
596 'val' => 'PURGED:$UNIXTIME$',
597 'ttl' => max( $ttl, 1 ),
598 'sbt' => true, // substitute $UNIXTIME$ with actual microtime
601 $ok = $this->relayer
->notify( "{$this->pool}:purge", $event );
603 $this->lastRelayError
= self
::ERR_RELAY
;
610 * Do the actual async bus delete of a key
612 * @param string $key Cache key
613 * @return bool Success
615 protected function relayDelete( $key ) {
616 $event = $this->cache
->modifySimpleRelayEvent( array(
621 $ok = $this->relayer
->notify( "{$this->pool}:purge", $event );
623 $this->lastRelayError
= self
::ERR_RELAY
;
630 * Check if a key should be regenerated (using random probability)
632 * This returns false if $curTTL >= $lowTTL. Otherwise, the chance
633 * of returning true increases steadily from 0% to 100% as the $curTTL
634 * moves from $lowTTL to 0 seconds. This handles widely varying
635 * levels of cache access traffic.
637 * @param float $curTTL Approximate TTL left on the key if present
638 * @param float $lowTTL Consider a refresh when $curTTL is less than this
641 protected function worthRefresh( $curTTL, $lowTTL ) {
642 if ( $curTTL >= $lowTTL ) {
644 } elseif ( $curTTL <= 0 ) {
648 $chance = ( 1 - $curTTL / $lowTTL );
650 return mt_rand( 1, 1e9
) <= 1e9
* $chance;
654 * Do not use this method outside WANObjectCache
656 * @param mixed $value
657 * @param integer $ttl [0=forever]
660 protected function wrap( $value, $ttl ) {
662 self
::FLD_VERSION
=> self
::VERSION
,
663 self
::FLD_VALUE
=> $value,
664 self
::FLD_TTL
=> $ttl,
665 self
::FLD_TIME
=> microtime( true )
670 * Do not use this method outside WANObjectCache
672 * @param array|string|bool $wrapped
673 * @param float $now Unix Current timestamp (preferrable pre-query)
674 * @return array (mixed; false if absent/invalid, current time left)
676 protected function unwrap( $wrapped, $now ) {
677 // Check if the value is a tombstone
678 $purgeTimestamp = self
::parsePurgeValue( $wrapped );
679 if ( is_float( $purgeTimestamp ) ) {
680 // Purged values should always have a negative current $ttl
681 $curTTL = min( -0.000001, $purgeTimestamp - $now );
682 return array( false, $curTTL );
685 if ( !is_array( $wrapped ) // not found
686 ||
!isset( $wrapped[self
::FLD_VERSION
] ) // wrong format
687 ||
$wrapped[self
::FLD_VERSION
] !== self
::VERSION
// wrong version
689 return array( false, null );
692 if ( $wrapped[self
::FLD_TTL
] > 0 ) {
693 // Get the approximate time left on the key
694 $age = $now - $wrapped[self
::FLD_TIME
];
695 $curTTL = max( $wrapped[self
::FLD_TTL
] - $age, 0.0 );
697 // Key had no TTL, so the time left is unbounded
701 return array( $wrapped[self
::FLD_VALUE
], $curTTL );
706 * @param string $prefix
709 protected static function prefixCacheKeys( array $keys, $prefix ) {
711 foreach ( $keys as $key ) {
712 $res[] = $prefix . $key;
719 * @param string $value String like "PURGED:<timestamp>"
720 * @return float|bool UNIX timestamp or false on failure
722 protected static function parsePurgeValue( $value ) {
724 if ( is_string( $value ) &&
725 preg_match( '/^' . self
::PURGE_VAL_PREFIX
. '([^:]+)$/', $value, $m )